The present invention concerns a method for manufacturing pyroelectric sensors by forming a thin pyroelectric film or layer on one face of the same wafer or substrate, particularly made of silicon. Each sensor is formed of several pixels each defined by a first electrode of its own located on one face of the pyroelectric film and a second electrode located on the other face of said film.
Such pyroelectric sensors have several uses, particularly for gas spectrometry and thermal imagining. They are relatively inexpensive. However, for the pyroelectric film to work properly, it is generally necessary for it to undergo electric polarisation, i.e. the application of an electric field between two electrodes which has the effect of orienting the dipoles of the pyroelectric film at least between the first and second electrodes of the pixels. By way of example, an electric field of approximately 30 V is applied across the first and second electrodes of the pixels with a temperature of the pyroelectric film of between 110 and 170xc2x0 C., for approximately 10 minutes. A plurality of sensors is batch manufactured with technology similar to that for the manufacture of integrated circuits. A silicon wafer of a diameter of 10 cm can contain several hundred sensors formed either of linear networks of pixels or two-dimensional networks in the form of pixel matrices.
A method for manufacturing such sensors has to allow at least pixel subsets to be polarised simultaneously; pixel-by-pixel polarisation having to be set aside for obvious reasons. Preferably, all the pixels of the same wafer are polarised simultaneously so as to reduce the manufacturing costs and the manufacturing time of the sensors. In order to polarise at least a pixel subset, on the one hand their upper electrodes and on the other hand their lower electrodes are electrically connected to each other. Generally, either the upper electrodes, or the lower electrodes are electrically connected to each other permanently so as to be set at a common potential during operation of the sensor. However, the other electrodes have to be electrically insulated from each other so as to provide elementary electric signals.
In general, those skilled in the art provide for the lower electrodes to be formed by a metallic film defining a common electrode for all the pixels. This allows manufacturing steps to be saved. Thus, the silicon wafer has a continuous metal film on which the thin pyroelectric film is deposited. The upper electrodes, which geometrically define the pixels, are deposited on this film.
In the aforementioned example, the upper electrodes of the sensor have to be insulated from each other prior to its use, as mentioned hereinbefore. However, in order to simultaneously polarise at least a pixel subset, temporary electric connections are formed between the upper electrodes. In other words, in order to carry out the polarisation the electrodes provided to supply the elementary electric pixel signals are also electrically connected. Thus, preferably, all these electrodes will be connected at least by subsets connected to at least one electric contact pad provided to carry out the polarisation.
However, such a method has a major drawback given that any accidental short-circuit between the electrodes of a pixel will cause a voltage drop for all the pixels of the subset concerned. Electric polarisation thus becomes impossible and a whole wafer may be wasted. Given the number of sensors manufactured on the same wafer, such an event causes considerable losses, which significantly increases the cost price of such sensors in industrial production.
The object of the present invention is to overcome the aforementioned drawback while allowing simultaneous electric polarisation for at least pixel subsets of a sensor wafer being manufactured.
This object is achieved by the subject of the invention as defined in claim 1 which concerns a method for manufacturing a plurality of pyroelectric sensors wherein there is provided a step where electric connections are formed between at least a subset of first electrodes each forming an electrode belonging to a pixel and wherein electric resistors are formed, arranged such that each electrode of said subset is series connected with one of said resistors.
As a result of the arrangement of the aforementioned electric resistors, a short-circuit at one pixel no longer causes a critical drop in the polarisation voltage applied during a polarisation step following the aforementioned step. Thus, it is possible to polarise the sensors of a wafer properly even in the presence of one or several short-circuits at certain pixels. The short-circuited pixels are of course inoperative in the operation of the manufactured sensors, but this problem remains localised only at the pixel that has been accidentally short-circuited.
So as to further reduce the risk of short-circuits and deterioration of the sensors, within the scope of the invention and according to a preferred embodiment, it is provided to also structure the electrodes intended to be connected to each other to a reference potential. Thus, the pixels are defined by lower and upper electrodes belonging to the pixels. This allows stray capacities formed by the electrodes to be reduced. These second electrodes are connected at least partially to each other by second electric connections, i.e. they are electrically connected at least by subsets preferably corresponding to the previously mentioned subsets.
According to sensor manufacturing variant, the first and second electric connections respectively connecting upper and lower electrodes intersect in projection onto the general plane of the wafer. To prevent any short-circuit between these first and second electric connections at the intersections causing deterioration of part of the sensors or all of them by a polarisation voltage drop, the first and second connections are arranged such that the intersections are located between the resistors and the electrodes to which they are connected.
According to a particularly advantageous embodiment, well suited for the linear pixel networks or for matrices with two lines of pixels, the connections between the electrodes associated with the protective resistors are automatically broken when the sensors are diced.